Chlorine dioxide (ClO2) is a potent oxidizing agent that is commonly used as a water treatment disinfectant. In the vapor phase, gaseous ClO2 ignites at concentrations greater than 10% by volume. Therefore, ClO2 cannot be shipped and must be prepared on site. Aqueous solutions of ClO2 generated at the point of use can be safely handled and applied as long as conditions that may lead to decomposition do not develop.
Chlorine dioxide is generally produced from sodium chlorite (NaClO2) and an acid, usually hydrochloric acid (HCl), which can be expressed by the chemical equation:5NaClO2+4HCl→4ClO2+5NaCl+2H2O.For most systems, about 80% of the sodium chlorite introduced into the generator is converted to ClO2 in excess acid, which is required to drive the reaction to completion.
Conventional acid-chlorite (“AC”) ClO2 generators use pumps or an eductor to properly flow and mix reactants in a preliminary reaction chamber. The generated ClO2 is diluted at the outlet motive water stream for either short-term storage or direct process application. Eductor-based systems provide safe operation because the reactor is under vacuum while ClO2 is generated. The combined vacuum and flow dynamics of the eductor prevent explosive levels of ClO2 vapor from forming by rapidly diluting ClO2 into the motive water supply. A high concentration of ClO2 is therefore not allowed to develop and persist in the reaction zone at elevated pressure. The motive water driving the eductor operation also promotes immediate ClO2 dilution, which prevents high concentrations of ClO2 from persisting or accumulating. In some embodiments, the motive water supply may be cycled between an “on” state and an “off” state to control ClO2 administration. Automated valves on each of the reactant precursor feed lines close to halt reactor operation when suitable motive water flow is not provided or process water flow is not detected.
Standard eductor operations require enough motive water flow to provide the appropriate suction force for the chemical feeds, and safe operational guidelines limit the final stream ClO2 concentration to no greater than 3,000 ppm. However, ClO2 concentration in the AC reaction chamber can greatly exceed this value, especially when acid is fed near stoichiometric ratio to chlorite and the reactor is not purged when the system is turned off. Higher reactant precursor concentrations will also elevate the hazards associated with these reactors; therefore, 7.5% sodium chlorite is typically paired with 10-15% HCl solution for these systems. Larger ClO2 generation capacity requires proportionately larger reaction chamber volumes, leading to increased operational hazards. Thus, AC systems are generally limited to lower production levels, such as 50 lb/day ClO2 or lower.
While the AC generator chemistry is commonly used in the industry and has many system design variations (see U.S. Pat. Nos. 7,128,879; 4,886,653; US 2002/0061263; and JP 2006 089332), most designs simply pump or educt the precursor chemical feeds into an isolated reaction chamber that then supplies the ClO2 product to an application point. In some cases, the reaction chamber is situated fully or partially within the process flow stream, but these systems often do not have mechanisms to prevent ClO2 generation in the event of low to no process flow, or to respond to a high-pressure event in the reaction chamber. Due to the desire for low-cost systems, the AC generator systems generally have minimal safety interlocks and limited ClO2 generation capacity.
U.S. Pat. No. 7,128,879 has incorporated float-dependent valve controls into the operation of a ClO2-generating system. These float sensors or float-dependent valves are used to dispense proper amounts of water and/or ClO2 chemical in response to levels detected in a basin or reservoir. While these float sensors may offer a way to ensure there is a proper amount of dilution water before chemical addition and preventing overflow of a basin, there is no safety mechanism attributed to the reaction chamber itself, which is where destructive failures can be the most severe due to the presence of high concentration of ClO2. U.S. Pat. No. 7,128,879 also pertains to treatment of a basin whereas the present invention seeks to treat an active process water supply line.